also put "dub siren" in youtube and you get all kinds of circuit bent weirdness.

But most of the above are all based on the classic NJD sirens that people expect from soundsystems-

But really there are tons of options, I have the JTS SE10, and we used to run it through a delay and it's perfect. We also added a moogerfooger for some more sophisticated modulations, and a cheap sampler (bosssp303 I think) to trigger other kinds of stuff.

Echo is key for any of them to sound nice really.

Some crappy 80's mixers even have them built in (a cheap siren noise), along with funny bomb noizes and gun shots too. We had an awesome one in west virginia someone had covered in tecate stickers that I miss dearly.